Inorganic black pigments are used in various fields, such as a pottery, a pigment, coloring of a resin and a pigment component of a glass paste. For example, a glass paste obtained by using a black pigment is used for a ceramic paste that configures a coating film of a circumferential edge portion of a window glass for a vehicle (see Patent Literature 1), and an insulating paste for insulating barriers of a plasma display (see Patent Literature 2), for example. Most of conventional black pigments contain chromium (Cr) as a constituent. Cr2O3 that is a chromium compound has been used heavily as one of starting oxide materials indispensable to manufacture a black pigment, such as being contained for improvement in heat resistance of a pigment, and adjustment of a color tone.
However, “the restriction of the use of certain hazardous substances in electrical and electronic equipment” are enforced now in EU for the purpose of minimizing danger affecting environments or human bodies in all stages from production to disposal of electricity and electronic equipment. Generally, this is referred to as RoHS Directive (the abbreviation for RoHS: Restriction of Hazardous Substances). In the RoHS Directive, use of hazardous substances is forbidden in principle and designated six substances are lead (Pb), mercury (Hg), cadmium (Cd), hexavalent chromium (Cr6+), polybrominated biphenyl (PBB), and polybrominated diphenyl ether (PBDE).
When carrying out product development pursuant to the RoHS Directive, ingredient components to be used for parts that constitute a product, and materials, for example, must be also thoroughly managed so that the above-mentioned six substances whose use is forbidden are not contained in a product. Regulation of hazardous chemical substances in view of such an environmental problem has been spread not only in EU countries but in every country in the world.
Cr generally contained as a starting oxide material of pigments changes to hexavalent chromium (Cr6+) having strong toxicity by addition of heat, for example. In manufacturing processes of pigments, flush processing, for example, is performed if necessary, so that hexavalent chromium generated is removed. However, chromium may be partially changed to hexavalent chromium again by passing through the drying step at about 180° C. For this reason, a black pigment per se which is a product may pose a problem. Moreover, depending on the use of a pigment, the pigment may be subject to heating and ultraviolet-ray exposure according to the operating condition. In such a case, a possibility that Cr contained in a black pigment is changed from trivalent (Cr3+) to hexavalent (Cr6+) by temporal change cannot be completely denied.
In the current RoHS Directive, only hexavalent chromium is taken as a target of regulation. However, when a product in which a black pigment containing chromium is used is disposed, safety with change of valency has been beginning to be regarded as questionable. Ultimately, attention to black pigments that fail to contain chromium component per se (see Patent Literature 3) has been increasingly paid. For the black pigment disclosed in Patent Literature 3, a strontium compound and ferric oxide are used as main components. Since solubility of strontium into water is high, a substantial manufacturing method is limited to a nonaqueous fluid or alcohol. Therefore, costs for manufacturing a pigment may increase, and range to be used and a use also may be considerably limited.
Furthermore, improvement is repeated to the combination and techniques used for manufacturing a black pigment, and a new black pigment which fails to contain chromium per se has been proposed (Patent Literatures 4 and 5). The black pigment disclosed in Patent Literature 4 is a pigment that contains oxides of Mn, Co, Ni and Fe as main components. The black pigment disclosed in Patent Literature 5 is a pigment that contains oxides of Mn, Fe, Cu and Co as main components. In particular, an excellent black color was obtained according to the black pigment disclosed in Patent Literature 5.
As described above, in connection with the necessity of management for the hazardous chemical substances contained in a product, a pigment containing a component that fails to contain chromium was able to be obtained. However, cobalt is contained as one of the main components, as described above. Cobalt is known as a cause of allergies and is desired to decrease out of composition of the main components as much as possible. Accordingly, it has been anxious for a new component-based pigment that fails to contain chromium in the main components from a view point of environment-friendly, and fails to contain cobalt, on the assumption that an excellent black color required for black pigments is present.